26 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



other hand, the more distal brachial, which bears the process, is strengthened on the 

 antipinnular proximal side by the posteriorly directed process, and thus becomes 

 broader there. 



It is evident that such a structure gives the arm increased capacity for flexion 

 laterally. An articulation in which both the end points of the transverse ridge are 

 strictly lateral evidently would not permit of any flexion at all laterally, whether the 

 articulation in other respects is oblique or not. 



The greater this reversion of the conditions of breadth in the proximal brachials, 

 the greater the capacity for lateral flexion in the arms. A certain amount of lateral 

 flexion is possible in all arms in which synarthries are developed. In fossil types where 

 synarthries are found and in recent stalked crinoids this restricted possibility for lateral 

 motion seems to be adequate. The same seems to be the case in the primitively 

 organized Atelecrinus in which no decrease at all of the breadth of the pinnular side of 

 the brachials appears in any part of the arms. Here one is able, from the primitive- 

 ness of the type in other respects, to assume an inconsiderable lateral mobility. An 

 increased lateral mobility here would, according to Gisl^n, most likely give rise to 

 discoidal brachials. 



Under certain conditions a case apparently similar to that of Atelecrinus may occur 

 because of enormously enlarged pinnule bases, as for instance hi the Calometridae. 

 In other cases the habit of creeping, as in a number of genera of Comasteridae, possibly 

 combined with original primitiveness, may bring about a similar phenomenon. 



In all the other comatulids reversion may be observed. In the cases in which it is 

 little developed the influence of the pinnule is about equal in importance to the counter- 

 acting, reversional, lateral flexibility; here we get in the proximal portion of the arms 

 a number of discoidal brachials. In the cases in which it is more strongly developed, 

 as for instance in the Antedonidae, the capacity for lateral flexion is the strongest, and 

 a number of brachials appear which are considerably narrower on the antipinnular 

 than on the pmnular side. 



THE MUSCLES 



Gislen's conclusions in regard to the origin and development of the musculature in 

 the crinoids are as follows: A pair of longitudinal muscle bands, corresponding to 

 those hi the holothurians, presumably existed on the ventral side of the arms of the 

 oldest crinoids. With the demand for increased and more rapid motion of the arms 

 these muscle bands began to play a more active part in flexing the arms, and therefore 

 acquired stronger attachments to the brachials. The brachials developed crests and 

 calcareous processes, and these extended outward into the longitudinal muscle band 

 and divided it into segments, which now correspond to the interbrachial muscles. 



For the only slightly movable arms of most of the Paleozoic crinoids the liga- 

 mentary articulations in the proximal part of the arms, which still lacked true muscles, 

 were sufficient for the flexion required. In these crinoids we therefore find only 

 syzygial, or more commonly more or less synostostical, articulations in the middle 

 and distal portions of the arms. Later, with the necessity for increased speed and 

 effectiveness in movement, the true muscles, which heretofore had only been found in 

 the ventrally situated soft parts, became associated with the calcareous skeleton. 

 This association took place earliest in the articulation between the radials and the 

 arms, standing alone in its type, in the Flexibilia, according to Springer becoming 



